Monoclonal anti-human monocyte antibodies OKM1 and OKM5 possess distinctive tissue distributions including differential reactivity with vascular endothelium
Monoclonal antibodies OKM1 and OKM5 detect unique antigenic determinants on functionally distinct subpopulations of human peripheral blood antigen presenting cells (APC). OKM1 reacts with the majority of large, adherent immune associated (Ia)+ peripheral blood monocytes/macrophages (M∅) required for soluble antigen-induced T cell proliferation, with a variable proportion of smaller nonadherent Ia- peripheral blood mononuclear cells that includes null cells and T(γ) cells, and with granulocytes. OKM5 also reacts with most large adherent Ia+ peripheral blood M∅ but does not react with the smaller nonadherent Ia- mononuclear cells or with granulocytes. Both the OKM1+OKM5+ and the OKM1-OKM5+ peripheral bood M∅ subsets are capable of triggering allogeneic T cell proliferation, but only the OKMI-OKM5+ population is capable of triggering the autologous mixed lymphocyte reaction (MLR). Endothelial cells (EC) and APC share several properties. EC express HLA-DR antigens, can be induced to express Fc and complement receptors, and are capable of replacing M∅ in soluble antigen-induced T cell proliferation and of stimulating in mixed lymphocyte reactions. These shared phenotypic and functional properties between EC and M∅ prompted us to investigate the reactivity of monoclonal antibodies OKM1 and OKM5 with EC and with other cell populations. For this purpose, we utilized an avidin-biothin complex immunoperoxidase technique in cryostat tissue sections of unfixed human tissues. We demonstrated that OKM1 and OKM5 detect a small proportion of tissue M∅, and exhibit distinctive distributions of reactivity with human tissues including pulmonary M∅ and EC. The majority of EC are OKM1-OKM5+, analogous to that small peripheral blood M∅ subset that possesses the capacity to present self antigens in the autologous MLR.